This invention relates to gold recovery, and more particularly to eluting gold from carbon filters.
It is well known in gold-using industries that activated carbon, e.g. coconut shell carbon, is useful for adsorbing gold from dilute solutions containing gold that might otherwise be discarded. U.S. Pat. No. 3,935,006 issued to D. D. Fischer on Jan. 27, 1976, and an improvement thereon, U.S. Pat. No. 4,208,378 issued to H. J. Heinen et al. on June 17, 1980 (both incorporated herein by reference) comprise the closest prior art known to Applicants relating to the instant invention.
Fischer introduces the idea of employing caustic-alcohol-water mixtures, containing relatively large percentages of alcohol, e.g. 40 to 100% by volume, for desorbing gold from activated carbon. Using Fischer's approach with eluants containing more than 25% by volume of water, "the efficiency of elution is sharply decreased".
Heinen et al. describe a method employing a much lower percentage of alcohol in the eluant solution, e.g. "preferably about 20 to 30% by volume", along with 1 to 2% (by weight) of sodium hydroxide and also sometimes containing "a small amount of sodium cyanide, e.g. about 0.02 to 0.1 percent (by weight) of the water solution". The approach of Heinen et al. also requires elution to occur at elevated temperatures, e.g. about 80.degree. to 90.degree. C. (i.e. 176.degree. to 194.degree. F.).
Both of the above-mentioned techniques have been successfully demonstrated to desorb 98% or more of the gold from loaded carbon. They both, however, include a limitation that seriously complicates practical implementation: both use relatively high levels of alcohol in the eluant. In fact, both show improved performance with higher levels of alcohol. Eluants with alcohol contents above about 3% are potentially explosive, especially at elevated temperatures (i.e. 100.degree. F. and above). Therefore, it is necessary to employ expensive special apparatus to prevent any eluant contact with air. It is quite inconvenient and requires expensive explosion-proof installations to prepare and use such hazardous materials. It is therefore an object of the instant invention to provide a safer and more efficient method for eluting adsorbed gold from carbon using an eluant containing only a relatively small amount of alcohol.